The present invention relates to backlights for instruments such as those using liquid crystal displays and, in particular, to a backlight suitable for avionics and providing a wide range of brightness in a color-balanced white output formed from the combination of light from multiple colored sources.
Graphic displays, such as those employing a liquid crystal display (“LCD”) screen provide a field of pixel elements each of which may be independently controlled to block or pass light, for example, from an underlying backlight.
A common backlight for use with an LCD screen provides a transparent panel edge-lit or backlit by one or more fluorescent tubes. In the edge-lit design, a reflective rear surface of the panel directs the edge illumination towards an LCD screen positioned against a front surface of the panel. The reflective rear surface of the panel may be gradated to produce an even field illumination behind the LCD compensating for an inherent falloff of brightness with distance of the fluorescent tube.
Fluorescent tubes provide a relatively high efficiency light source providing a broad color spectrum output suitable for backlighting color LCD screens in which pixels associated with red, green, and blue light components must be evenly illuminated for good color rendition.
When backlit LCD screens are used in avionics applications, a wide range of illumination output is desirable to allow the avionics display to be easily readable, both in bright sunlight and in levels of very low light and over a wide range of ambient temperatures. In low light situations, too much illumination can interfere with dark adaptation and night vision goggles or similar equipment.
Fluorescent tubes have a number of disadvantages in avionics applications including: the need for a high voltage power supply, a fragility of the glass tube, a tendency to fail unexpectedly, low efficiency at low ambient temperatures, and a limited ability to change brightness level. For these reasons, it is known to use light-emitting diodes (“LEDs”) as a replacement for fluorescent tubes, particularly in avionics and other demanding applications. In order to provide a multi-spectral output needed for color LCD screens, such LED backlights provide clusters of red, blue, and green LEDs. Preferably, each color of LED may be separately controlled in brightness. When these different colors of LEDs are energized together with the correct relative brightness, they produce a light that appears substantially white to the human eye.
The relative brightness of each of the LEDs must normally be adjusted electronically to obtain the correct color balance to provide white light. Maintaining this color balance as the backlight is varied in brightness, can be difficult because of different and often non-linear relationships between light output and current for each of the different colors of LEDs. That is, over a given range, a uniform change in current provided to the LEDs for each color will tend to cause a color shifting of the backlight. The function relating brightness to current can change with the temperature and age of the LED further complicating attempts to maintain color balance over a wide range of illumination.